Urea tank and base plate with an integrated heating element

ABSTRACT

Urea tank and base plate with an integrated heating element in which the heating element comprising at least one resistive track affixed to one flexible film and/or placed between two flexible films.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional application of U.S. application Ser. No.12/599,478, filed Mar. 5, 2010, which is a U.S. National StageApplication under 35 U.S.C. §371 of International Application No.PCT/EP2008055936 filed May 15, 2008, which claims priority to FrenchApplication No. 0755118 filed May 16, 2007, and to French ApplicationNo. 0756635 filed Jul. 20, 2007, these applications being incorporatedherein by reference in their entirety for all purposes.

The present application relates to a urea tank with an integratedheating element, and also to a base plate intended for such a tank andthat integrates the heating element.

Legislation on vehicle and heavy goods vehicle emissions stipulates,amongst other things, a reduction in the release of nitrogen oxidesNO_(x) into the atmosphere. One known way to achieve this objective isto use the SCR (Selective Catalytic Reduction) process which enables thereduction of nitrogen oxides by injection of a reducing agent, generallyammonia, into the exhaust line. This ammonia may derive from thepyrolytic decomposition of an ammonia precursor solution, whoseconcentration may be the eutectic concentration. Such an ammoniaprecursor is generally a urea solution.

With the SCR process the high levels of NO_(x) produced in the engineduring combustion at optimized efficiency are treated in a catalyst onexiting the engine. This treatment requires the use of the reducingagent at a precise concentration and of extreme quality. The solution isthus accurately metered and injected into the exhaust gas stream whereit is hydrolysed before converting the nitrogen oxide (NO_(x)) tonitrogen (N₂) and water (H₂O).

In order to do this, it is necessary to equip the vehicles with a tankcontaining an additive (generally urea) solution and also a device formetering the desired amount of additive and injecting it into theexhaust line.

Given that the aqueous urea solution generally used for this purpose(eutectic 32.5 wt % urea solution) freezes at −11° C., it is necessaryto provide a heating device to liquefy the solution in order to be ableto inject it into the exhaust line in the event of starting in freezingconditions.

Several systems have been provided in the prior art for this purpose.Generally, these systems comprise relatively expensive heating devicesthat involve either specific heating elements or a bypass of the enginecooling circuit (for example, see Application WO 2006/064001 in the nameof the Applicant).

As regards the use of specific heating elements, it is known to putheating tubes inside the tank containing the urea solution, andoptionally in the urea (feed and return, if necessary) lines. However,these tubes have a minimum diameter of several mm which makes themdifficult to bend and compromises their insertion into certain parts.

One currently known alternative concerns the insertion of cartridge(rod) heaters at various locations in the tank. Although thesecartridges (rods) may be mobile (articulated) relative to one another,the same problem exists as regards the flexibility of these cartridges.

Thus, the Applicant has observed that a problem arises at theconnections of the urea lines to the tank. This is because a plug of ice(frozen solution) tends to form in them, and given that they aregenerally injection-moulded parts made of plastic of a certainthickness, heating via an external heating tube is not effective.However, these connections are generally formed from thin parts(otherwise known as “delivery tubes”) into which a heating tube cannoteasily be inserted.

Application FR 06/07531 in the name of the Applicant aims to solve thisspecific problem by providing a hollow connection equipped, since itsmanufacture, with an integrated heating filament, at least one part ofwhich extends freely inside the passage provided for the liquid. Such anarrangement makes it possible to ensure that the liquid is in directcontact with a large heating surface area, and therefore to heat itrapidly.

However, this is an arrangement specific to heating this location(connection), said filament possibly not easily being used to heat thewhole of the tank. Moreover, it cannot easily be inserted into otheraccessories, considering its reduced flexibility.

The present invention especially aims to solve this problem by providinga liquid tank equipped internally with a particular heater, theflexibility, shape and heating power of which may easily be adapted toany geometry of the tank or of its accessories.

Therefore, the present application relates to a urea tank with anintegrated heating element in which said heating element comprises atleast one flexible heating part (that is to say, equipped with a deviceenabling it to heat up, such as a resistive element for example).Preferably, this flexible part is a flexible heater, that is to say thatit comprises at least one resistive track inserted between two flexiblefilms or affixed to a flexible film (for example, see U.S. Pat. No.7,268,325 which describes such heaters and their methods ofmanufacture). Most particularly preferably, the heating element ispredominantly composed of a flexible heater.

Although such “flexible heaters” have been commercially available formany years, to date no one has considered their use in urea tanks, thisbeing the case even though the use of this type of heater in the ureaapplication has many advantages which will be explained in detail lateron.

The term “urea” is understood to mean any, generally aqueous, solutioncontaining urea. The invention gives good results with eutecticwater/urea solutions for which there is a standard quality: for example,according to the standard DIN 70070, in the case of the AdBlue® solution(commercial solution of urea), the urea content is between 31.8% and33.2% (by weight) (i.e. 32.5+/−0.7 wt %) hence an available amount ofammonia between 18.0% and 18.8%. The invention may also be applied tothe urea/ammonium formate mixtures, also in aqueous solution, sold underthe trade name Denoxium™ and of which one of the compositions(Denoxium-30) contains an equivalent amount of ammonia to that of theAdblue® solution. The latter have the advantage of only freezing from−30° C. onwards (as opposed to −11° C.), but have the disadvantages ofcorrosion problems linked to the possible release of formic acid. Thepresent invention is particularly advantageous in the context ofeutectic water/urea solutions.

The urea tank according to the invention may be made from any material,preferably one that is chemically resistant to urea. In general, this ismetal or plastic. Polyolefins, in particular polyethylene (and moreparticularly HDPE or high-density polyethylene), constitute preferredmaterials.

This tank may be produced by any conversion processes known in the caseof hollow bodies. One preferred processing method, in particular whenthe tank is made of plastic, and in particular HDPE, is theextrusion-blow moulding process. In this case a parison (in one or moreparts) is obtained by extrusion, and is then shaped by blow moulding ina mould. One-part moulding of the tank from a single parison gives goodresults.

This tank is advantageously equipped with a base plate or mounting plate(that is to say a support having substantially the shape of a plate)onto which at least one active accessory of the urea storage systemand/or injection system is attached. This base plate generally has aperimeter, closed up on itself, of any shape. Usually, its perimeter hasa circular shape.

In a particularly preferred manner, this base plate is a submergedmounting plate, i.e. that seals an opening in the lower wall of thetank. The expression “lower wall” is in fact understood to mean thelower half of the tank (whether or not it is moulded in one piece orfrom two parison sheets or cut-outs). Preferably, the base plate islocated in the lower third of the tank, and more particularlypreferably, in the lower quarter, or even squarely in the bottom of thetank. It may be partly on the lower side wall, in which case it isslightly slanted once mounted in the vehicle. The location and/ordirection of the base plate especially depends on the location of thetank in the vehicle, and on the space occupied around it (consideringthe components to be integrated therein).

This base plate therefore incorporates at least one component that isactive during storage and/or injection. This is understood to mean thatthe component is attached to or produced as one part with the baseplate. This component may be incorporated inside the tank, or on theoutside with, if necessary, a connection (delivery tube) passing throughit.

Preferably, the base plate according to this variant of the inventionintegrates several active storage and/or metering components and, moreparticularly preferably, it integrates all the active components whichare brought to be in contact with the liquid additive found in, leavingfrom or arriving into the additive tank.

Preferably, the component is chosen from the following elements: a pump;a level gauge; a temperature sensor; a quality sensor; a pressure sensorand a pressure regulator. These elements each have an active role in theadditive storage and/or injection system, and the fact of integratingthem into a mounting plate that is “submerged” (that is to say, alwaysin the presence of the additive, as long as the tank is not empty) hasspecific advantages:

-   -   for the level gauge: the low point is better defined and        therefore the measurement is less affected by deformations of        the tank;    -   for the sensors of temperature, quality or other        characteristic(s) of the additive: makes it possible to easily        locate it in the critical zone for feeding to the injection        system.

The level gauge may be of any type. Preferably, it is a gauge with nomoving parts, for example of the capacitive type.

The base plate of the tank according to this variant of the inventionmay be obtained by any known means, but as a preference it is obtainedby injection moulding, this method making it possible to obtain gooddimensional accuracy.

Preferably, the base plate is based on a material that is resistant tourea and to the products created during ageing of the latter, such aspolyacetal, and in particular POM (polyoxymethylene); polyphthalamides(for example, AMODEL® grades); or polyamides, and in particular gradesof polyamides that are reinforced (for example with glass fibres).Preferably, the base plate is made of polyamide and it comprises adelivery tube moulded as one piece with it as described previously.

The tank according to the invention incorporates (that is to say, isequipped with) a heating element having at least one flexible part. Thequalifier “flexible” is in fact understood to mean “easily deformable”,this generally being in a reversible manner. Generally, this correspondsto a flexural rigidity (defined as being equal to (Eh³)/(12(1−ν²)) whereE is the Young's modulus of the flexible part measured according to theASTM D790-03 standard, h is its thickness and ν is the Poisson's ratioof its constituent material) below 4000 N·m; preferably, in the contextof the invention, the rigidity of the flexible part is less than orequal to 1000 N·m, or 100 N·m or even 10 N·m and most particularlypreferably, less than or equal to 1 N·m.

Preferably, this flexible part is a flexible heater, that is to say aheater comprising one or more resistive track(s) affixed to a film orplaced between two films (that is to say two substantially flatsupports, the material and thickness of which are such that they areflexible). This film is preferably made of a plastic (although any otherinsulating material may be suitable) and, in particular, is based on anelastomer. For ease, reference will subsequently generally be made toseveral resistive tracks. In this variant, for the flexural rigiditycalculation defined above, the Poisson's ratio will preferably be takento be that of the constituent material of the film or films.

The resistive tracks may be based on metal, carbon, etc. or even acombination of such conductive materials. They are generally metallic(and most particularly preferably, made of a urea-resistant metal suchas a stainless steel). They are generally sandwiched between twoflexible films. They are applied (for example, by jet printingtechniques) onto a flexible film and then covered with another flexiblefilm or overmoulded using an insulating (preferably elastomeric)material. The two films are then firmly attached together (for example,by vulcanization) to ensure sealing around the resistive tracks. Thesetracks are preferably connected in parallel so that if one of the tracksis damaged, it does not impede the operation of the other tracks.

The flexible films may be made of silicone resin, polyolefin(polyethylene or polypropylene), thermoplastic elastomer (or TPE),polyester, polyimide (such as the KAPTON® resin), etc. Preferably, theyare based on silicone, polyolefin or TPE considering the fact thatpolyester and polyimide have a lower resistance to urea, particularly athigh temperature.

The flexible films may also comprise several superposed layers ofresistors (resistive tracks). They may also comprise a glass-fibrecoating to improve their mechanical strength.

Stainless steel resistive tracks sandwiched between two silicone resinfilms, one of which is covered with a network of glass fibres, give goodresults in the context of the invention.

With a view to avoiding the problems of overheating which may occur whenthe heater cannot dissipate its power by conduction and which may leadto the degradation of the heater and/or urea, it may be advantageous tolocally control its specific power, that is to say to locally vary thepower loss per unit area of resistive track by altering the length andwidth of the resistive track. For example, if the width and length of aresistive track applied to a given surface are doubled, its totalresistance and the power dissipated by this track will remain unchanged.On the other hand, the total surface area of the track will bemultiplied by 4, leading to a specific power (or power loss per unitarea of track) divided by 4. This therefore results in a lower tracktemperature at this location.

One advantage of flexible heaters is that, due to their flexibility,they can easily be inserted into cavities of any shape and whilesubstantially conforming to the walls, even when following a tortuouspath. Furthermore, they may have any shape, possibly even a complicatedshape, so that a single heater (designed according to a sort of“pattern”) may be used to heat various parts of the tank, even partsthat are distant from one another. Thus, in one preferred variant, theflexible heater comprises a body and at least one excrescence ortentacle equipped with at least one part of resistive track. Mostparticularly preferably, it comprises several excrescences or tentacleswhich are positioned uniformly in the tank so as to be able to heat itscontents completely and as homogeneously as possible, even in the nooksdistant from the body of the heater from which the tentacles extend. Inthis variant, the body of the heater may comprise a resistive track andtentacles, at least one other resistive track, these tracks preferablybeing connected in parallel to a power supply terminal.

The term “body” is understood to mean a part of the heater where thetentacles start from or where the resistive tracks start from and arriveat. The abovementioned tentacles may be obtained in any known manner.Advantageously, they are the product of cuts made in the heater itselfwithout interrupting the resistive track or tracks. One particular wayof proceeding consists in placing the resistive tracks onto a siliconefilm according to a preestablished pattern; in covering the tracks witha second silicone film; and finally in carrying out the aforementionedcuts.

According to the invention, the flexible heater is located inside theurea tank, and is therefore submerged (partially or completely) in theurea (liquid phase) when the tank is full. Preferably, this heater ispositioned so as to be able to heat the urea even when the tank isalmost empty. For this purpose, it is advantageous for at least onetentacle to have a sufficient length so that it can rest on the bottomof the tank. One variant with free flexible tentacle(s) (i.e. ones thatare not attached to the tank) has the advantage that the flexible heaterassembly and its support may be suitable for many different urea tankdesigns since the tentacles may be folded up inside these tanks andadapt to their various shapes.

As mentioned above, the flexible heater according to the invention ispreferably affixed to a support (preferably one that is rigid, i.e. cannot be deformed very much) which also has a protective role towards saidheater (and/or certain tank accessories). Generally, the body of theheater is attached to the support and the tentacles are either free orattached to certain tank parts or accessories.

This support may be any part that may or may not fulfil another role inthe tank. Advantageously, this support is integrated into (produced froma single part with or attached onto) a base plate such as describedabove. Preferably, this is a submerged base plate such as defined abovewhich makes it possible to do without withdrawing line(s) (that connectsaid base plate and the bottom of the tank) which must be heated, whereappropriate. Alternatively, it might be a conventional base plate placedon the upper wall of the tank, but in this case it is necessary to heatthe lines and also to provide a sufficient length (of the tentacles,where appropriate) to be able to heat the bottom of the tank.

Preferably, the support and the base plate are produced from a singlepiece by injection moulding of plastic such as described above.

The support may have any shape. Advantageously, it has a substantiallycylindrical shape and extends substantially over the entire height ofthe tank. The term “substantially” is in fact understood to mean that itextends over the entire height generally to within a few mm (or even 1to 2 cm), this being so as to take into account manufacturing tolerancesand/or thermal expansions. Therefore, in the case of freezing, there isa risk of forming a layer of ice that cannot be heated in this zone,associated with which is a risk of the injection system stoppingfollowing the vacuum created under this ice layer and which prevents theurea from being pumped.

Therefore, it is advantageous to provide the heater with at least oneflexible tab comprising at least one part of resistive track, and ofwhich the location, size and shape are such that this tab is permanentlyin contact with the upper wall of the tank so as to pierce a shaft inthe ice layer described previously.

It should be noted that this concept of a heating element permanently incontact with the upper wall of the tank is also advantageous outside thecontext of the invention, with any type of heater and/or liquid that iscapable of freezing in its storage tank where it is withdrawn bypumping.

Preferably, the heater support delimits a volume (closed or open) insidewhich at least one part of the flexible heater is attached. In theadvantageous variant with tentacles, the body of the heater is attachedin this volume and the tentacles extend at least partly outside thisvolume.

In one variant, this volume is closed, that is to say is delimited by asubstantially solid side wall that acts as a urea trap (ensuring aminimum volume of liquid urea under all usage conditions). In thisvariant, it is particularly advantageous that the trap be integratedinto a submerged base plate, that is to say be produced from one piecewith (or attached to) this base plate, which avoids having to heat aline connecting these two components. The term “substantially solid” isunderstood to mean that this wall may comprise orifices, but these arethen in an upper part so as to be able to trap urea in its lower part.

In this case, if the heater is equipped with tentacles, the latterpreferably extend into the tank from the upper edge of the trap (whichis itself generally open).

Alternatively, the support may comprise a sort of box comprisingopenings which make it possible to create convection loops thatfacilitate heating of the urea. Some of these openings areadvantageously used to allow the tentacles to extend towards the bottomof the tank. It should be noted that this convection is also promoted bythe movements of the urea.

It should be noted that these 2 variants (urea trap and convection box)may be combined (as a trap with orifices in its upper part as mentionedpreviously).

In particular, when the tentacles extend from the top of the support,there is a risk that the free end of certain tentacles floats, or eventhat it extends into the gas phase which must not be heated. To avoidthis, it is advantageous to provide a sort of float that is firmlyattached to these ends and that forces them to remain in the liquidphase.

The principle of the float may be generalized to all shapes of flexibleheaters: attaching the “rising” part(s) of the heater to a (some)float(s) makes it possible to ensure that the heater always remainsunder this (these) float(s) and therefore submerged, which avoidsheating the vapour zone located above the liquid urea.

The heater is preferably attached to its support mechanically usingclips, rivets, bolts, etc. that pass through orifices made for thispurpose both in the heater and in its support. Other orifices may bemade in the heater, for example to provide an entry for probes.

At least one of the aforementioned tentacles is advantageously sized soas to be able to be inserted into a tube (in particular, asmall-diameter tube), for example into a delivery tube for connection tothe abovementioned urea injection circuit. Preferably, at least twotentacles are provided in order to be inserted into the urea supply andreturn tubes, respectively. Most particularly preferably, the end ofthis (these) tentacle(s) is equipped with a relief that makes itpossible to get hold of it and pull it through the tube/delivery tube.One relief which is particularly suitable is a simple orifice throughwhich a hook may be inserted.

According to one particularly advantageous variant of the invention, theheater is designed so as to be able to follow the entire hydraulic pathof the urea, that is to say to extend from the filling interface of thetank to the delivery tube for connection to the injection system,passing through the filter, where appropriate, the venturi, the gauge oreven the pump. Thus, according to one advantageous variant of theinvention, a single heater heats the whole of the tank and itsaccessories. This has the advantage of limiting the electricalconnections, as explained previously.

Independently of the context of the invention then (that is to say withany type of heater and/or liquid capable of freezing), it isadvantageous to provide a heater at the filling interface (pipe) sinceplugs of ice are capable of being formed at this location and ofpreventing the tank from being filled in the event of freezing.Therefore, the heater according to the invention preferably comprises atleast one tentacle that extends at least partly into a filling interfaceof the tank.

As regards the particular case of heating the filter, it should be notedthat the latter is generally manufactured separately and inserted intothe urea injection circuit (that is to say connected to a line thatforms part of this circuit), generally at the base plate, by a simplelaektight mechanical attachment involving a seal. Therefore, theflexible heater cannot be inserted into the zone downstream of thefilter due to the risk of impairing the impermeability.

A 1^(st) method of solving this problem consists in providing a by-passin this zone (where the seal is), that is to say providing an additionaltube that has an inlet orifice for the heating element just before thefilter and an outlet orifice for this element just after the filter.This solution is, however, complicated since it is imperative that thisadditional tube is rendered leaktight around the heater in order toprevent the urea from by-passing the filter.

Therefore, a preferred solution consists in overmoulding the seal ontothe heating element before assembling said heating element in the tankand before connecting the filter. Again, this solution is alsoadvantageous outside the context of the invention, for example with afilament-type heating element and/or a liquid other than urea.

In the advantageous variant described previously, according to which theheater comprises at least one resistive track inserted between twoflexible films or affixed to one flexible film, one particularlyadvantageous variant consists in providing at least 2 tracks which maybe connected in parallel in order to form a capacitive gauge that makesit possible to measure the level of urea in the tank. These tracksgenerally have a low resistance and participate little in the heatingeffect of the heater.

In one variant, 4 such tracks may be provided so as to form, in pairs, areference capacitor and a measurement capacitor respectively.Preferably, these tracks are positioned in such a way that the referencecapacitor is submerged in the urea as soon as there is some (be thisonly a little) in the tank (which amounts to saying that it ispreferably located on the bottom of the tank) and in such a way that themeasurement capacitor covers the entire height of the tank in themeasurement zone.

Given the specificity of the heating and level measurement tracksrespectively, it is advantageous that their respective widths and alsothe thickness of the surface film, where appropriate, are adapted.Generally, the measurement tracks are thinner and are covered with afilm of lesser thickness. The two types of tracks may be produced fromstainless steel (preferably from a grade which has a substantiallyhigher urea resistance than that of Cu for example), be affixed to acommon silicone film that has a sufficient thickness to act as asupport; and comprise a coating also based on silicone resin, which isthinner on the measurement tracks (e.g. around 100 μm) than on theheating tracks (where the thickness may reach 1 mm).

Although described in the context of the present invention (heating andlevel measurement in a urea tank), it is obvious that the flexibleheater described above is also suitable for other types of tanks sinceit allows the saving of a gauge regardless of the liquid for which thetank is intended.

The present invention also relates to a base plate such as describedpreviously, that is to say that integrates at least one active accessoryof the urea storage system and/or injection system and, in the contextof the invention, that integrates at least one heating element having aflexible part preferably consisting of a flexible heater (and preferablyone only, but designed and positioned so as to be able to heat the wholeof the tank and the accessories integrated into it).

All the preferred variants relating to the flexible heaters and to thebase plates mentioned previously apply to this aspect of the invention,especially:

-   -   the fact that the heater may comprise several resistive tracks        preferably connected in parallel;    -   the fact that it is preferably equipped with tentacles, of which        some are free to extend into the tank and others are equipped        with orifices enabling them to be inserted more easily into a        line, delivery tube, etc.; and/or    -   that the support for heating it can act as a fuel trap and/or a        convection box.

The present invention is illustrated nonlimitingly by the appended FIGS.1 to 4.

FIG. 1 consists of a 3-dimensional view of a base plate with a flexibleheater illustrating several variants of the invention.

FIG. 2 consists of a 3-dimensional view of another base plate with aflexible heater also illustrating several variants of the invention.

FIG. 3 consists of two 3-dimensional views of a base plate with aflexible heater according to yet another variant of the invention. InFIG. 3 b, the urea trap has been deliberately erased in order to see theshape of the heater in it.

FIG. 4 illustrates a top view and two cross sections through this samebase plate.

In the figures identical numbers denote identical or similar components.

The base plate (1) illustrated in FIG. 1 integrates a flexible heater(2) comprising a body (3) fixed to the inside of a support (4) usingclips (5) and tentacles (6) connected to the body (3) and having a freeend capable of extending inside a urea tank (not represented).

The support (4) of the heater (2) comprises openings (7) that make itpossible to create convection loops and to allow the free end of thetentacles (6) to pass through.

The heater comprises a tab (8) intended to be in permanent contact withthe upper wall of the tank to create a shaft through the cap (layer) ofice as explained previously.

It also comprises a tentacle (6′) specially sized to be able to beinserted into a delivery tube (9) for connection to the urea injectionsystem (not represented).

The base plate (1) incorporates a gauge (10) for measuring the level ofurea in the tank, and also other accessories which do not appear clearlyin this figure, such as a temperature sensor, a filter, etc.

One of them is firmly attached at its end to a tube (11) connected to aventuri (not represented) and enabling liquid to be sucked up outside ofthe support (4) in order to fill the latter. In this variant, thesupport (4) is free from orifices and acts as a liquid (urea) trap.

The base plate (1) illustrated in FIGS. 3 and 4 is, on the other hand, abase plate intended to seal an orifice in the upper wall of a tank. Forthis purpose it is equipped with a screw thread (1′) (visible in thecross sections from FIG. 4) able to cooperate with a complementary screwthread on an excrescence of the tank.

This base plate (1) also incorporates a support (4) for a flexibleheater (2) acting as a urea trap. This support (4) is attached to thebase plate (as can be seen in the cross sections from FIG. 4) and ispinned to the bottom of the tank by at least one spring (notrepresented).

The heater (2) is attached to the base plate (1) and to the bottom ofthe support (4).

The flexibility of the heater (2) enables it to adapt to the variouscompression ratios of the spring that depend on the distance between thetop and bottom of the tank.

The invention claimed is:
 1. A base plate suitable for a urea tank that integrates at least one active accessory of a urea storage system and/or injection system and also at least one heating element having at least one flexible heating part, wherein the flexible heating part is a flexible heater comprising at least one resistive track placed between two flexible films and which: comprises several resistive tracks connected in parallel; is equipped with tentacles, of which some are free to extend into the tank and others are equipped with orifices enabling them to be inserted more easily into a delivery tube or a line; or is affixed to a support capable of acting as a urea trap and/or a convection box.
 2. A base plate according to claim 1, that integrates one heating element.
 3. A urea tank comprising a base plate according to claim
 1. 4. A base plate according to claim 1, wherein the at least one resistive track is affixed to a flexible film.
 5. A urea tank comprising a base plate according to claim
 4. 